A powerful U.S. airborne force was deep inside Iraq Monday, more than halfway to the Euphrates River, threatening Baghdad's main supply route to occupied Kuwait, allied military sources said.

If they reach the river, an artery to the heart of Iraq, they will be in a position to assault President Saddam Hussein's elite Republican Guard from the north as it faces Kuwait, and other allied forces, to the south.So far, the 101st Airborne Division, spearheaded by scores of attack helicopters, has met only light resistance, the sources said.

That indicated either that surprise was complete or that Saddam, his forces in the Kuwaiti theater pounded from land, sea and air by the biggest invasion force assembled since World War II, was unable to respond.

The Euphrates River valley could be the key to a quick ending of the 40-day war. It runs from Basra where southern Iraq abuts Kuwait, north toward the Iraqi capital, Baghdad.

Some military analysts say that by driving deep into Iraq, American forces could isolate the Republican Guard, the main Iraqi military threat to U.S.-led coalition forces, cutting them off from reinforcements from the Baghdad region where Saddam has his military headquarters.

The 101st Airborne, part of the lightly armored, fast-moving 18th Army Corps, struck into Iraq far to the west of the Kuwaiti border in what officers called the biggest helicopter-borne assault in history.

"Some very light elements have moved quite a long way north from the west and have not been in contact with Iraqi forces," one source said.

The most heavily armored battle corps yet created, the 7th with 1,300 tanks and tens of thousands of men, also crossed into Iraq.

The 7th, normally based in Germany, was anxious not to outrun the supply lines for its M1A1 Abrams tanks and other heavy equipment, the sources said.

If the 7th meets the Republican Guard and the later measures up to its reputation and fights, the world could see one of its biggest tank battles in 50 years.